1. Field of the Invention
This invention relates to a continuous distillation apparatus and method and, more particularly, to a continuous distillation apparatus and method wherein the rectifying section and the stripping section are configurated as separate chambers with the rectifying section being operated at a higher temperature than the stripping section and heat pipes are used to transfer thermal energy from the rectifying section to the stripping section.
2. The Prior Art
Distillation is the single most important separation unit operation in the process industry. It is widely used to upgrade feed stocks, separate reaction intermediates, and purify products in processes ranging from cryogenic separation of oxygen, nitrogen, and helium to the recovery of aromatics from coal. In a recent study of the distillation processes in a number of refineries and chemical plants, distillation was determined to be an important energy consumer in almost every refinery and chemical plant. In petroleum refineries, which are the largest energy consumers on a per plant basis, crude and vacuum distillation alone accounts for between 22.5 and 51% of the total energy consumed. Accordingly, any enhancement of the energy balance for a distillation unit operation will have a significant impact on a wide cross section of the process industry.
In the conventional distillation column, heat is supplied in a reboiler and removed through a condenser with the operating temperature of the reboiler being substantially higher than the operating temperature of the condenser. Accordingly, thermal energy is introduced into the distillation column at the highest temperature end (the reboiler), and removed from the distillation column at the lowest temperature end (the condenser). Because of this temperature differential between the reboiler and the condenser, the separation of components is always accompanied with a degradation of energy even when heat leaks and other losses are excluded. Therefore, the conventional distillation process, when viewed as a thermodynamic process, is notoriously inefficient. Values as low as 1.9% have been reported for the thermodynamic efficiency of industrial distillation columns.
With particular reference to FIG. 1, a typical, prior art, continuous distillation or fractionating column equipped with the necessary auxiliary equipment and containing rectifying and stipping sections is shown. The column A is fed near its center with a feed of definite concentration. The feed is assumed to be a liquid at or near its boiling point. The plate on which the feed enters is called the feed plate. All plates above the feed plate constitute the rectifying section, and all plates below the feed, including the feed plate itself, constitute the stripping section. The liquid in the feed flows down the stripping section to the bottom of the column where a definite liquid level is maintained.
Liquid is removed from the stripping section and flows by gravity to reboiler B that generates vapor and returns the vapor to the bottom of the stripping section portion of the column. The reboiler may be of the kettle (as shown), thermosyphon, forced circulation, etc., type. The bottom product may undergo-further processing or may flow (as shown) through a cooler H which also preheats the feed by heat exchange therewith.
Vapor from reboiler B passes up the entire column through both the stripping and rectifying sections and is removed from the column and partially or totally condensed in a condenser C. The condensate is collected in an accumulater D from which reflux pump F removes liquid and delivers it to the top plate of the rectifying section. This liquid stream is called reflux and provides the down-flowing liquid in the rectifying section necessary to act on the up-flowing vapor. Reflux liquid provides the required rectification since no rectification would occur in the rectifying section without the reflux. Without rectification the concentration of the overhead product would be no greater than in the vapor rising from the feed plate. Condensate from accumulator D that is not picked up by the reflux pump F is withdrawn as overhead product and may be cooled in a heat exchanger E, called the product cooler. If no azeotropes are encountered, both overhead and bottom products may be obtained in any desired purity if enough plates and adequate reflux are provided.
It should be emphasized again that thermal energy is removed from the vapor in condenser C at a lower temperature than the thermal energy introduced into reboiler B. Accordingly, in the absence of any external device, it is not possible to transfer the low temperature thermal energy removed from condenser C into the higher temperature environment of reboiler B.
The distillation system shown in FIG. 1 is usually simplified, especially in general chemical laboratories. For example, in place of the reboiler, a heating coil is placed in the bottom of the column and is used to generate vapor from the pool of liquid there. The condenser may also be placed above the top of the column with the reflux pump and accumulator omitted. Reflux then returns to the top plate by gravity while a special valve, called a reflux splitter, is used to control the rate of reflux return while the remainder of the condensate forms the overhead product.
Various methods for increasing the thermodynamic efficiency of the distillation process are known and are discussed in numerous publications and include, for example, vapor recompression, vapor reuse, secondary reflux, and split tower configurations. Additional information regarding conventional distillation apparatus may be found in Unit Operations of Chemical Engineering, McCabe, W. L., and Smith, J. C., McGraw-Hill Book Company, Inc., New York, 1956.
Additional information on fractional distillation, including more efficient apparatus, can be found in a publication Elements of Fractional Distillation, Robinson, C. S. and Gilliland, E. R., fourth edition, McGraw-Hill Book Company Inc., New York (1950).
Historically, energy supplies from fossil fuels (notably natural gas and petroleum) have been both cheap and plentiful, and it has, therefore, been difficult to justify the additional capital equipment and fabrication costs necessary to enhance the thermodynamic efficiency of the distillation column. However, recent increases in the cost of energy (particularly that derived from natural gas and petroleum) has placed an increased emphasis on enhancing distillation column thermodynamic efficiency or providing alternative low energy separation techniques that formerly could be dismissed summarily on economic grounds.
A new distillation scheme using secondary reflux and vaporization to enhance thermodynamic efficiency is discussed in a paper "Distillation with Secondary Reflux and Vaporization: A Comparative Evaluation" by Mah, R. S. H., Nicholas, Jr., J. J., and Wodnik, R. B., AIChE Journal, Volume 23, Number 5, September, 1977 (pages 651-658).
In view of the foregoing, it would be a significant advancement in the art to provide a continuous distillation/separation apparatus and method whereby thermal energy is transferred from the rectifying section to the stripping section in a continuous distillation apparatus. Such an apparatus and method based on the use of heat pipes is set forth herein.